Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive surface is then exposed to a light image corresponding to the copy desired to be reproduced. This exposure records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed by bringing a developer mixture into contact therewith. A common type of developer comprises carrier granules having toner particles adhering triboelectrically thereto. The two-component mixture is brought into contact with the photoconductive surface, where the toner particles are attracted from the carrier granules to the latent image. This forms a toner powder image on the photoconductive surface which is subsequently transferred to a copy sheet. The toner powder image is then heated to fuse it to the output sheet.
The ionographic printing process also produces an electrostatic latent that is subsequently developed, transferred and fused. However, in the ionographic process the latent image is produced on an insulating charge receiving member. The charge receiving member collects the charge, in the form of charged ions, which are output from an ion generating print head in response to an image intensity signal.
When electrophotographic or ionographic printing systems are used, it is generally necessary to monitor and regulate the mass of toner which is transferred to the latent electrostatic image. This is important to control not only the quality of the prints made by the systems, but also to enable adjustment of those subsystems which are affected as a result of the amount of toner used to develop an image. Furthermore, the monitoring and control requirements are multiplied in modern multicolor printing machines. For example, U.S. Pat. No. 3,960,444 to Gundlach et al. (Issued Jun. 1, 1976) and U.S. Pat. No. 4,660,059 to O'Brien (Issued Apr. 21, 1987), both of which are hereby incorporated by reference, disclose multicolor electrophotographic and ionographic printing machines, respectively. Various approaches have been devised to estimate and control toner concentration in the developer or the amount of toner used to develop an electrostatic latent image, the following disclosures appear to be relevant:
U.S. Pat. No. 3,409,901 PA0 Patentee: Dost et al. PA0 Issued: Nov. 5, 1968 PA0 U.S. Pat. No. 4,065,031 PA0 Patentee: Wiggins et al. PA0 Issued: Dec. 27, 1977 PA0 U.S. Pat. No. 4,721,978 PA0 Patentee: Herley PA0 Issued: Jan. 26, 1988 PA0 U.S. Pat. No. 4,847,659 PA0 Patentee: Resch, III PA0 Issued: Jul. 11, 1989 PA0 U.S. Pat. No. 4,908,666 PA0 Patentee: Resch, III PA0 Issued: Mar. 13, 1990 PA0 A Toner Dispensing Control System PA0 by Alfred M. Loeb PA0 Xerox Disclosure Journal, Vol. 6, No. 6 (Nov./Dec. 1981)
The relevant portions of the foregoing patents and disclosure may be briefly summarized as follows:
U.S. Pat. No. 3,409,901 discloses a xerographic system in which a toner concentration control system feeds toner to the developing mechanism in proportion to the area and density of the print. A cathode-ray tube (CRT) is used to expose a photoconductive member, and the signal which drives the CRT is also provided to a toner feed signal means where the signal is summed. When the signal exceeds a predetermined level an output signal is generated to cause toner to be dispensed into the developer mechanism.
U.S. Pat. No. 4,065,031 describes a device for regulating the dispensing of toner particles to a developer mix. During the operation of an electrostatographic printing machine a sensing mechanism, including a photosensor for determining the density of toner developed on a photoreceptor, outputs signals indicative of the toner concentration. The signals are summed and processed to determine if additional toner should be added to the developer mix.
U.S. Pat. No. 4,721,978, the relevant portions of which are hereby incorporated by reference, discloses an apparatus for controlling the concentration of toner particles used to form a highlight color document. Three signals are generated and processed to regulate the dispense rate of toner particles used to form the highlight color portion of the output document. The first signal is an indication of the percentage of the document area arranged to have color highlighted portions thereon. The second signal corresponds to the rate of toner particle usage per document, as determined by a central processing unit, and the third signal indicates the number of copies to be produced. To determine the amount of highlight color toner used, the three signals are multiplied, the product of the signals being used as a control signal which corresponds to the required dispense rate.
U.S. Pat. No. 4,847,659 describes an electrostatographic machine which replenishes toner in a developer mix in response to a toner depletion signal which represents the toner usage rate. The toner depletion signal is determined from the number of character print signals applied to a print head, or in other words, the number of pixels to be toned. The depletion signal is used in conjunction with a second signal, which represents a proportional toning contrast, such that the constant of proportionality between the toner depletion signal and a toner replenishment signal is adjusted according to the second signal.
U.S. Pat. No. 4,908,666 teaches a toner replenishment control structure which operates in one of two control states to control contrast characteristics when using developers having two developer materials. The first developer material exhibits contrast characteristics which vary with concentration and the second developer material does not exhibit contrast variation due to concentration variance. The system has a first control state for replenishing the first developer material as a function of a concentration signal and a second control state for replenishing the second developer material as a function of a contrast signal.
Loeb describes a toner dispensing control system that relies upon an intensity signal, representing the intensity of light reflected from the surface of an original document, and a developed density signal to produce an error signal. Subsequently a combination signal is produced as a function of the error signal, in accordance with a predetermined algorithm, to control the dispensing of toner to the developer material.